How can you tell the severity of a food allergy? How do I know what my LO is allergic to?!?

I'm pumping and feeding my little one milk, and he has had diarrhea for the past three weeks +. I have cut milk out completely from my diet, along with soy, and at times it seems like it is working because he will have two-three normal diapers, then right back to diarrhea (even when i haven't eaten anything different between feedings!!!). Has this happened to anyone? Does anyone have any advice for me?! Help! How do I know if this is what he is allergic to?!